Comment by JeremyNT

I once found a nice open source ambient noise generator on F-Droid. Fully local, offline.

One day the developer decided to switch to a pay SaaS model. They updated the open source app to be a thin client for their web service.

Surely many existing users on the play store found quite a surprise when they updated!

F-Droid at least provides a little protection here, with the independent builds and easy downgrades, and if the community is big enough you see forks appear. But sadly this kind of cashing in is always going to be a risk with open source software in app stores.